Method of forming photoresistive coatings and composition



Nov. 25, 1958 w. HEIMANN 2,861,903

METHOD OF FORMING PHOTORESISTIVE commas AND COMPOSITION Filed Nov. 3. 1953 INVEN R Mm W $4.4 6. M M

2,861,903 Ice Patented Nov. 25, 1958 METHOD OF FORMING PHOTORESISTIVE COATINGS AND COMPOSITION Walter Heimann, Wiesbaden, Germany, assignor to Societe Nouvelle de LOutillage R. B. V. et de la Radio- Industrie (R. B. V.-R. 1.), Paris, France, a joint-stock company Application November 3, 195a, set-m No. 390,045

Claims priority, application Germany November 10, 1952 9 Claims. (Cl. 117-200) The present invention is related with photoelectric layers'of the photoresistive type. It is well known that semiconductors such as cadmium sulfide show internal photosensitivity when they are activated with heavy metal such as copper, silver, gold, mercury, nickel, etc., their photosensitivity in the visible spectrum is greater than the sensitivity of any other known material working under identical conditions. Nevertheless, the photosensitivity of a cadmium sulfide layer is very small in the red part of the visible spectrum and practically null in the near infra-red part of the spectrum.

It is usual practice to obtain crystals of cadmium sulfide by reaction in the vapor phase between cadmium vapors and hydrogen sulfide with an excess of hydrogen. The crystals obtained by this method are not very homogeneous due to the variations of concentration of the different reagents at different points.

A known remedy for this lack of homogeneity is to proceed to a reheating of the crystals. During this process, it is very difiicult to control with precision the ratios of activators and as is well known photoelectric properties are very sensitive to slight differences in ratios.

On the other hand, this process makes it very difficult to introduce in the base material any additive activator other than an excess of cadmium which is of no use as far as infra-red sensitivity is considered.

It has been proposed to make infra-red sensitive cells with cadmium selenide or telluride. The sensitivity of these cells is rather high in the near infra-red but very poor in the visible part of the spectrum.

The main object of the invention is to obtain photoresistive layer very sensitive, both in the visible spectrum, the near ultra-violet, and the near infra-red.

It is known that the cadmium sulfide cells activatedwith copper show rather quick decrease of sensitivity with time. This decrease of sensitivity is already noticeable when the cells are vacuum-tight and kept in vacuum, but the rate of decrease is much larger when the cells are in contact with the surrounding atmosphere and humidity. This loss of sensitivity is probably due to two different causes: first a flow of the copper atoms through the crystalline array, which phenomenon appears also in the activated lead sulfide layer and, secondly, an oxydacopper sulfide.

tion of the copper which is accelerated by the presence of water or water vapor.

It is a second object of the invention to make a wide frequency photosensitive cell, the sensibility of which is constant with time.

It is also known to use as an infra-red sensitive material lead sulfide, but the sensitivity is very poor.

It is a third object of the invention to provide an infra-red sensitive photoresistive material, the sensibility of which is 10 times larger than the lead sulfide sensitivity and which supplies an output power of a few milliwatts without the use of any amplifier.

According to the invention, the photoresistive material is made of a base material plus an additive, said base activated cadmium sulfide.

2 material being cadmium sulfide and said additive being one of the following materials, or a mixture of two of them: selenium, tellurium, cadmium selenide and cadmium telluride.

The relative weightof additive is 5 to 30% of the weight of CdS. The photoelectric properties of said material are obtained by means of any of the heavy metal activator introduced in the microcrystalline layer by the thermal diffusion process.

The invention will be best understood by reference to the following description of one way of manufacturing an infra-red sensitive photoresistive layer and a cell incorporating said layer. To obtain 20 photoresistive layers of about 1 cm. in surface the following operations are made. 1

Commercially available pure cadmium sulfide is vacuum evaporated on 20 inert plates of 1 cm. in surface. These plates are made of glass and cleaned before the evaporation takes place. The thickness of the sulfide coating is of a few microns. The coated plates are then removed from the vacuum tight oven and embedded in '100 grammes of a mixture of powders of microcrystalline highly purified cadmium sulfide to which has been added 10 grammes of pure selenium. The mixture of powders comprises pure cadmium sulfide powder and highly The activator concentration in the activated part of the powder is calculated so that the final concentration in the whole of the powder is equal to the activator concentration which is to be obtained in the photoresistive layer. When the particular activator used is copper sulfide and the concentration which is to be obtained in the layer is 10* in weight, the powder is made of grammes of pure cadmium sulfide powder and 10 grammes of pure selenium and 10 grammes of cadmium sulfide containing 0.1 gramme of When two different activators are used, the mixtures of powders are made of three different powders plus the additive, one powder being pure cadmium sulfide, and the two others of highly activated cadmium sulfide comprising each one a concentration of a particular activator such that the final concentration in the mixture is equal to the concentration which is to be obtained in the photoresistive layer. The preparation of small quantities of highly activated powders is usually more easy than the preparation of a large quantity of powder with a very small concentration of activators.

The powders aremixed together and put in a closed and air-tight crucible made of material which will not I react with sulfur or cadmium or any of the activators.

An alumina crucible is satisfactory. The coated plates are buried in the mass of the powder mixture and the crucible is put in an oven where it is heated in air at a temperature between 500 and 600 C. The volume of the oven is such that the correct amount of oxygen is in contact with the powders.

The photoresistive properties of the coating are checked from time to time and the heating is continued until the desired sensitivity in the desired range of wavelength is obtained. The coated plates are then removed from the powders in which they are embedded.

During heating a thermal diffusion between solids takes 3 In the example of manufacture which has been described oxygen is used as one of the activators and it is supplied by the air in which the crucible is heated. When eng n a tua or. is. us d. he. he tin hould. e pe fo d' i i n ae t tm h su rea. ats a r itrogen.

When-the concentration of the additive to the cadmium describedf The photosensitive layer'is' shown at l coat-. ing aglass plate 2. One of the electrodes in contact with the layer is shown at 3. It consistsof a conductive path painted with ametalliepaint on the uppersurface of'the layer and whichis continuedalong-th'e sideand under-.

neath plate 2- so as to-malr'e an electric contact with spring 4-which-rests in a housing 5 of'the externalpin 6. There are two such electrodes on the sensitive layer the second of which is in contact with external pin 7. The operating voltage is applied between the two pins 6 and 7.

An envelopesurrounds the active part-ofthe cell. It comprises an upper window 8 which-is transparent in the wavelength rangecorresponding-to-the sensitivity ofthe layer. The window is fixed "in-theupper part of -the envelope. The lower part 100i the-envelope is sealed to the upper part. Both are-made ofplastic and colored, for instance by painting, so that nolight falls-on the layer apart from the lighttransmitted through the window. In the lower part of the envelope are provided two threaded holes in which'are secured the threaded ends of the external pins.6and 7. The electric contact betweenally encountered-with .copperactivated cadmium sulfide:-

cells, on the..contrar.-y, the..sensitivity shows a slight increase. during-the firstmonth of storage. This characteristicsholdsnboths for vacuum tight. cells and for cellsin. which:.the..photosensitive;.material 'is in. contact with. the atmosphere...

It is usual .toapply.aworking-potential to thistype of.-

cell up to,.10Q-v.- without any .noticeable deterioration. of the photoresistive coating with time. high intrasred sensitivity it is possible to obtain from such cells as described above, an-output power of a few milliwatts=withoutany amplification. This is a great technical advantage since it leads to very simple and inexpensive apparatus. As is well known theinfra-redcells made until now require the use of a low level D; C."- amplifier which is always delicate to design.

Owing to the What I claim is:

l". A photoresistive material sensitive to near infrared, visible and near ultra-violet light consisting essentially of cadmium sulfide as a basic component, at least one additive material selected; from the group consisting of selenium, tellurium, cadmium selenide and cadmium telluride, and at least one metallic activator selected from the group consisting of copper, silver, gold,

mercury, and nickel, the concentration of saidadditives' beingbetwccn Sand 30%v in weight with respect tothe basic'component, and theconcentration of the activator material is of the order of 10- in weight.

2. A photoresistive material according to claim 1 wherein said additive materials are selenium and telluriurn, and said metallic activator is introducedlin said material by thermaldifiusion between solids.

3. A photoresistive material according to claim 1 hcreinsaid-a ditiv materials. arecadmiumselcnide-and cadmium telluride, and said; metallic. activator; isintro:

dn ed in sa id mat 1 'i3l by, thermal diffusion between-sol:-

ids.

4. A- photoresistive material, according to claim, 1-. wherein said additive materials are selenium and, tellurium, and-,said metallic activator material consists. of two metals selected from the class specified.

5. A photoresistive material according to claim. 1-, wherein .said additive. materials are cadmium selenideand cadmium telluride, and said metallic, activator material consistsof two metals selected from the group specified.

6. Themethodof forming photoresistive ,material sensitive to. near. infra-red, visible and ,near, ultra-violet light, whichponsists. in forming uponthe surface of a dielectricplate a film of cadmium sulfide by vacuum evapo-- ration, said film being of a thickness of a few microns, placing said film.in.contact with a mixture .of microcrystalline powders consisting of parts of pure cadmiurn sulfide powder, 10 parts of pure selenium, and 1.0 parts of cadmium sulfide containing 0.1 part of copper sulfide, and heating said film andnsaidpowders to a tem: perature between 500 and 600 C.

' 7. A method according to claim 6 wherein said heat-v ing .step is carried out in thepresence of oxygen.

8; The method of forming photoresistive material sensitive to near infra-red, visible and near ultra-violet light, which consists in forming upon, the surface of a dielectric plate a film ofcadrniurn sulfide by vacuum evaporation, said film being of a thickness of afcw microns, placing said film in contact with a mixture of microcrystalline powders consisting of '90 parts ofv pure cad-l mium selenide andcadmium telluride, and l0'parts of cadmium sulfide containing 0.11 part of at least one metallic activator selected from the group consisting of copper, silver,- gold, mercury, and nickel, and heating said film and said powders to a temperature between 500 and 600 -C.'

9: A method according-to claim 8 wherein said heating:step-iscarried outin the presence of oxygen.

References-Cited in the-file of this'patent UNITED STATES PATENTS 

6. THE METHOD OF FORMING PHOTORESISTIVE MATERIAL SENSITIVE TO NEAR INFRA-RED, VISIBLE AND NEAR ULTRA-VIOLET LIGHT, WHICH CONSISTS IN FORMING UPON THE SURFACE OF A DIELECTRIC PLATE A FILM OF CADMIUM SULFIDE BY VACUUM EVAPORATION, SAID FILM BEING OF A THICKNESS OF A FEW MICRONS, PLACING SAID FILM IN CONTACT WITH A MIXTURE OF MICROCRYSTALLINE POWDERS CONSISTING OF 90 PARTS OF PURE CADMIUM SULFIDE POWDER, 10 PARTS OF PURE SELENIUM, AND 10 PARTS OF CADMIUM SULFIDE CONTAINING 0.1 PART OF COPPER SULFIDE, AND HEATING SAID FILM AND SAID POWDERS TO A TEMPERATURE BETWEEN 500 AND 600*C. 